Low contact resistance metal contact

ABSTRACT

A semiconductor structure and methods of making the same. The semiconductor structure includes a substrate having a suicide region disposed above a doped region, and a metal contact extending through the silicide region and being in direct contact with the doped region.

FIELD OF THE INVENTION

The invention generally relates to integrated circuit design andfabrication and, more particularly, to a semiconductor structure havingimproved contact resistance and methods of making the same.

BACKGROUND OF THE INVENTION

Semiconductor device performance is dependent upon numerous factors, onebeing the total device resistance. The total device resistance is, inturn, a function of parameters such as contact resistance, wiringresistance, channel resistance, etc. Decreasing the total deviceresistance can improve device performance (i.e., improve device speed).

As CMOS devices scale further downward, the contact resistance becomes ahigher portion of the total resistance due to the fact that channelresistance decreases while metal contact resistance increases with thescaling trend. The contact resistance is dictated by the contact area,barrier height, and silicon doping concentration. However, contact areais constrained by any given scaling rule, and doping concentration isalready at a maximum level in current CMOS technology. The barrierheight of metal contacts, however, remains a viable avenue forimprovements in contact resistance.

One type of material commonly employed in fabricating metal contacts ismetal silicides, such as Cobalt silicide or Nickel silicide. Cobaltsilicide, and other metal silicides, are typically fabricated using aconventional self-aligned silicide (salicide) process, wherein a blanketTiN/Co film is deposited over the devices and annealed to form Cobaltmonosilicide over the exposed silicon regions (source, drain and gate)of transistors. A selective wet etch is employed to remove the TiN capand the non-reacted Cobalt left over the oxide or nitride regions. TheCobalt monosilicide is then subjected to a second anneal which convertsthe monosilicide into a Cobalt disilicide layer. Such Cobalt suicidesand Nickel suicides are referred to as mid-gap metals because theirworkfunction is between the workfunction of n-type and p-typesemiconductors. They can be used as a contact metal for both n-type andp-type devices with certain contact resistance.

In know structures, a contact stud is formed over the silicide contactby first etching a trench through an interlayer dielectric (and,possibly, through an optional etch stop nitride layer) to the topsurface of the silicide contact. The trench is subsequently filled withmetal (e.g., tungsten by CVD, or copper by electroplating) to form acontact stud that is in direct contact with the silicide contact.

To further reduce total resistance, it has been suggested to usedifferent silicide materials for the respective n-type and p-type areasof a circuit. However, this is generally not a financially suitablesolution since it adds many additional processing steps to the overallfabrication scheme.

Accordingly, there exists a need in the art to overcome the deficienciesand limitations described hereinabove.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is a semiconductor structurecomprising a substrate comprising a silicide region disposed above adoped region, and a metal contact extending through the silicide regionand being in direct contact with the doped region.

In a second aspect of the invention, there is a method comprisingproviding a substrate comprising a silicide layer above a doped region,forming a trench through the silicide layer to expose a surface of thedoped region, and forming a silicide contact on the surface of the dopedregion.

In a third aspect of the invention, there is a method comprising forminga mask over a first portion of a dielectric material, and removing asection of the dielectric material from a second portion of thedielectric material not covered by the mask. The method furthercomprises forming a first trench through the first portion of thedielectric material to a first silicide layer disposed above a firstdoped region. A second trench is formed through the second portion ofthe dielectric material and through a second silicide layer, therebyexposing a surface of a second doped region. A silicide contact isformed on the surface of the second doped region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 5 show process steps and intermediate structuresaccording to aspects of the invention; and

FIGS. 6 through 10 show process steps and intermediate structuresaccording to further aspects of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is directed to semiconductor devices and methods of makingthe same. More particularly, aspects of the invention providesemiconductor devices in which the contact resistance is improved byreducing the barrier height of the contact. This is achieved, inembodiments, by exposing doped semiconductor material at the bottom ofthe contact trench and forming low contact resistance suicide directlyonto the doped semiconductor material. In this manner, the barrierheight of the contact is reduced, thereby reducing the contactresistance and improving device performance.

FIG. 1 shows an intermediate structure during the fabrication of asemiconductor device. The intermediate structure comprises a substrate10, shallow trench isolation (STI) 15, and a conventional type gate 20.Formed on either side of the gate 20, and within the substrate, aredoped (source/drain) regions 25, 26, and silicide regions 30, 31.Disposed over the silicide regions 30, 31 and gate 20 are an etch-stopnitride layer 35 and an interlayer dielectric 40. The structure may beeither an n-type or p-type structure, depending upon the materials usedand the intended application of the finished device.

The intermediate structure of FIG. 1 may be formed in any suitablemanner, using conventional processes and materials. For example, thesubstrate 10 may comprise a semiconductor substrate, which is typicallya silicon substrate, a buried oxide (BOX) layer formed on the substrate,and a semiconductor layer, which is typically a silicon layer, formed onthe buried oxide layer. The semiconductor layer may be comprised ofvarious semiconductor materials, such as, for example, Si, SiGe, SiC,SiGeC, etc. The substrate 10 may be fabricated using techniques known tothose skilled in the art. For example, the substrate 10 may be formed byconventional bonding and cutting processes, or alternatively, aconventional separation by implantation of oxygen (SIMOX) process.

Similarly, techniques known in the art may be utilized to form the STI15. A conventional technique entails patterning with a photoresist,etching the trench, chemical vapor deposition (CVD) of oxide to fill thetrench, and planarizing the surface such as by chemical mechanicalpolishing (CMP).

Plural STIs 15 may be employed, resulting in the formation of silicon oninsulator (SOI) islands. In implementations, each island may accommodatea gate device. For example, a first SOI island may accommodate a p-typedevice, and a second, possibly adjacent, SOI island may accommodate ann-type device. It is understood, however, that any number of islands maybe formed, and any configuration of p-type and n-type devices may beemployed.

Still referring to FIG. 1, the gate 20 may comprise a standard gateformed in any conventional manner. For example, the gate 20 may befabricated by first forming gate dielectric on an exposed surface (e.g.,top) of the SOI island. The gate dielectric layer may be formed by aconventional deposition process such as CVD or plasma-assisted CVD, or athermal growing process such as oxidation, nitridation oroxynitridation. The gate dielectric may include any device qualitydielectric material such as an oxide, nitride, oxynitride or anycombination and multilayer thereof. The thickness of the gate dielectricis not critical to the present invention.

A gate stack may be formed on the gate dielectric utilizing aconventional deposition process such as CVD, plasma-assisted CVD orplating. The gate stack may include a gate material such as polysilicon,amorphous silicon or other materials suitable for MOSFET gatecomposition. The gate material may be formed on the surface of the gatedielectric utilizing conventional deposition processes well known in theart such as, for example, CVD or plasma-assisted CVD. An optionaldielectric-capping layer (not shown) may be present atop the gatematerial. When present, the optional dielectric-capping layer maytypically be comprised of an oxide, nitride or oxynitride and formedutilizing a conventional deposition process such as, for example, CVD orplasma-assisted CVD. Alternatively, a conventional thermal growingprocess such as, for example, oxidation, may be used in forming anoptional dielectric-capping layer.

Following formation of the gate stack on the gate dielectric layer, thegate stack and gate dielectric layer may be subjected to a conventionalpatterning process which includes lithography and etching steps. By wayof example, the lithography step may entail applying a photoresist,exposing the photoresist to a pattern of radiation, and developing thepattern utilizing a conventional resist developer. Following thelithography step, a conventional etching process such as reactive-ionetching, plasma etching, ion beam etching or laser ablation may beemployed in transferring the pattern to the gate stack and the gatedielectric.

Spacers may be formed along gate sidewalls. For example, spacer materialsuch as a nitride (e.g., Si₃N₄) may be deposited in a conventionalmanner, such as by chemical vapor deposition (CVD) using a silanesource. Other techniques, which may be suitable for deposition of anitride layer, include low-pressure CVD (LPCVD) and atmospheric pressure(CVD) (APCVD). Portions of the deposited nitride layer are subsequentlyetched away in a conventional manner to form the spacers.

Still referring to FIG. 1, the doped (source/drain) regions 25, 26 maybe formed in a conventional manner. For example, a conventionalimplantation or out-diffusion process may be employed to selectivelydope the source and drain regions with appropriate ions known to thoseof skill in the art.

Silicide regions 30, 31 may be formed over the doped (source/drain)regions 25, 26 in any suitable manner. For example, the silicide regions30, 31 may be formed by selectively sputtering a Cobalt (or Nickel) filmonto the source and drain regions 25, 26, and annealing the film to forma Cobalt (or Nickel) silicide. In embodiments, the silicide has athickness of about 20 nm to 40 nm, although other thicknesses may beused within the scope of the invention.

Etch stop nitride layer 35 and interlayer dielectric 40 may also beformed using conventional techniques and materials. For example, theetch stop nitride layer may be formed by any suitable deposition method.The interlayer dielectric 40 may be composed of silicon dioxide formedfrom silane and nitrogen sub-oxide by plasma-assisted CVD with highdensity plasmas.

Embodiments of the invention provide an improved contact resistance byreducing the barrier height of the contact region. For example, as shownin FIG. 2, a trench 60 may be formed to extend not only through theinterlayer dielectric 40 and etch stop nitride 35, but also through thesilicide region 30 to the top of the doped (source/drain) region 25. Thetrench 60 preferably only extends through the silicide region 30 toexpose the top surface of the doped (source/drain) region 25, but may beover-etched (i.e. extend slightly into the doped region 25) to ensurethat no silicide remains in the bottom of the trench 60. The trench 60may be formed using a conventional reactive ion etch (RIE).

In implementations, after formation of the trench 60, a layer 65 ofmetal is deposited onto the exposed portions of the structure, as shownin FIG. 3. For example, a conventional sputtering process may be used todeposit the layer 65. In embodiments, the layer 65 has a thickness ofabout 10 nm to 50 nm, although other thicknesses may be employed.

According to aspects of the invention, the metal that is deposited toform layer 65 is chosen based upon the type of device being created. Forexample, for PFET's where the doped (source/drain) region 25 comprisesp-type semiconductor material, the layer 65 may comprise Platinum (Pt)or Iridium (Ir). For NFET's where the doped (source/drain) region 25comprises n-type semiconductor material, the layer 65 may compriseErbium (Er) or Ytterbium (Yb).

As shown in FIG. 4, the structure is annealed to form a low contactresistance silicide 70 at the bottom of the trench 60 in direct contactwith the doped (source/drain) region 25. In implementations, theannealing takes place in an inert gas (e.g., Ar, N₂, He, etc.) at atemperature of about 200° C. to 500° C., although other gasses andtemperatures may be used. Moreover, the anneal may be performedsingularly or combined with an other anneal step later in thefabrication.

Referring to FIG. 5, desired portions of the non-reacted portions of thelayer 65 are stripped using wet chemicals in a conventional salicide(self-aligned-silicide) process. In embodiments, a contact stud 75 isformed in the trench 60 in any suitable known manner, such as, forexample, via tungsten deposition or copper electroplating. Unwantedmaterial may be removed and the top surface planarized with a CMPprocess.

As seen in FIG. 5, the resulting structure has a low contact resistancesilicide 70 in direct contact with the doped (source/drain) region 25.When the low contact resistance silicide 70 comprises Erbium (Er) orYtterbium (Yb) on n-type silicon, the barrier height is reduced to about0.25 to 0.30 eV, resulting in a contact resistance of about 50 to 80ohm-μm. This shows a marked improvement over the barrier height andcontact resistance of conventional mid-gap metals, thereby providing animproved semiconductor device. The benefit of this process is especiallysignificant when applied on a narrow width device such as an SRAM devicewhere contact area is a significant part of the total source and drainarea.

FIGS. 6 through 10 show process steps for forming a device according tofurther aspects of the invention. As shown in FIG. 6, an NFET 90 and aPFET 95 are arranged side-by-side. Similar to the structure shown inFIG. 1, the NFET 90 comprises doped (source/drain) regions 100,101 andsilicide regions 102,103, and the PFET 95 comprises doped (source/drain)regions 105, 106 and silicide regions 107, 108. A common etch stopnitride 110 and interlayer dielectric 115 may extend over both the NFET90 and PFET 95, and an STI 116 may be disposed between the NFET 90 andthe PFET 95.

As shown in FIG. 7, a mask (e.g., photoresist) 120 is disposed over theinterlayer dielectric 115 above the PFET 95. Any suitable photoresist120 may be utilized. Then, as depicted in FIG. 8, a section of theinterlayer dielectric 11 5 above the NFET 90 is removed, such as, forexample, using a conventional etch process. In embodiments, the depth(i.e., thickness) of the removed section equals or slightly exceeds thedepth (i.e., thickness) of the silicide region 102, which, inembodiments, is about 20 nm to 40 nm. After the etch, the mask (e.g.,photoresist) 120 is removed using known processes. In alternativeembodiments, the mask may be formed over the NFET (instead of the PFET,as described above), thereby effectuating removal of a section of theinterlayer dielectric over the PFET.

As depicted in FIG. 9, first trench 125 and second trench 126 areformed. In embodiments, the trenches 125,126 are substantiallyconcurrently formed using a single etch (e.g., RIE) process. Due to thesubstantially concurrent formation using the common etch, the trenches125, 126 have substantially the same depth. However, due to the priorremoval of a section of the interlayer dielectric 115 above the NFET 90,the first trench 125 extends through silicide region 102 into directcontact with the doped (source/drain) region 100, while second trench126 extends to the top of, or slightly into, the silicide region 107.

Referring to FIG. 10, in embodiments, a layer 130 of metal is formed onthe exposed surfaces of the structure, such as, for example, by asputtering process similar to that described with respect to FIG. 3. Inthis manner, the bottoms of both trenches 125, 126 are substantiallyconcurrently covered with a layer 130 of deposited metal. In the exampleshown, because the NFET 90 has its trench 125 in direct contact with thedoped (source/drain) region 100, the layer 130 preferably compriseseither Er or Yb. In alternative embodiments, where the PFET has itstrench extending into direct contact with the doped (source/drain)region and the NFET does not, the layer of metal preferably compriseseither Pt or Ir. In either case, the layer 130 has a thickness of about10 nm to 50 nm, although other thickness and materials may be usedwithin the scope of the invention.

FIG. 10 shows the structure after an annealing step, which may besimilar to that described with respect to FIG. 4. The annealing resultsin the formation of a low contact resistance silicide 135 at the base ofthe first trench 125 in direct contact with the doped (source/drain)region 100. The rest of layer 130 remains un-reacted by the anneal.Thus, a new silicide is not formed at the base of the second trench 126during the anneal.

FIG. 10 further depicts the metal contact studs 140, 141 formed in thetrenches 125, 126. The contact studs 140, 141 may be formedsubstantially concurrently using a common deposition process (e.g., forTungsten) or electroplating step (e.g., for Copper). Lastly, the topsurface of the structure may be planarized using a CMP.

The resulting structure, shown in FIG. 10, has a low contact resistancesuicide contact 135 at the NFET 90 and a conventional mid-gap contactformed at the PFET 95. In this manner, the NFET 90 is provided with animproved (i.e., reduced) contact resistance. Moreover, because thecontact assemblies for the NFET 90 and PFET 95 are formed using commonetching, sputtering, annealing, and metal fill steps, the improvementprovided to the NFET 90 does not require as many extra steps as wouldnormally be required in implementing different silicide materials forthe respective n-type and p-type areas.

The semiconductor device as described above may be part of the designfor an integrated circuit chip. In embodiments, the chip design iscreated in a graphical computer programming language, and stored in acomputer storage medium (such as a disk, tape, physical hard drive, orvirtual hard drive such as in a storage access network). If the designerdoes not fabricate chips or the photolithographic masks used tofabricate chips, the designer transmits the resulting design by physicalmeans (e.g., by providing a copy of the storage medium storing thedesign) or electronically (e.g., through the Internet) to such entities,directly or indirectly. The stored design is then converted into theappropriate format (e.g., GDSII) for the fabrication ofphotolithographic masks, which typically include multiple copies of thechip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

The method as described above is used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

While the invention has been described in terms of embodiments, thoseskilled in the art will recognize that the invention can be practicedwith modifications and in the spirit and scope of the appended claims.

1-21. (canceled)
 22. A method of making a semiconductor structure,comprising: forming a mask over a first portion of a dielectricmaterial; removing a section of the dielectric material from a secondportion of the dielectric material not covered by the mask; forming afirst trench through the first portion of the dielectric material to afirst silicide layer disposed above a first doped region, therebyexposing an upper surface of the first silicide region; forming a secondtrench through the second portion of the dielectric material and througha second silicide layer, thereby exposing a surface of a second dopedregion; forming a silicide contact on the surface of the second dopedregion; and substantially concurrently filling the first and secondtrenches with contact stud metal, wherein: the first trench and thesecond trench are substantially concurrently formed, a depth of theremoved section of the dielectric material equals or exceeds a depth ofthe second silicide layer, the forming the silicide contact comprisesdepositing a metal on exposed surfaces of the first trench and thesecond trench and annealing the metal, the first doped region comprisesa n-type semiconductor material, the second doped region comprises anp-type semiconductor material, and the depositing the metal comprisessputtering a film of Platinum or Iridium.